The present invention relates to a liquid crystal display panel, and more particularly to an improvement for obtaining better display quality in a horizontal electric field liquid crystal display panel, typified by an in-plane switching (IPS) mode liquid crystal display panel.
A liquid crystal display panel is an electro-optical device for displaying images in which an electric field is formed in a liquid crystal layer that is sandwiched between a pair of substrates and the intensity of the light transmitting through the liquid crystal layer is controlled by varying the orientation of liquid crystal molecules in the liquid crystal layer.
Systems for applying electric field across the liquid crystal layer are classified into two types: a so-called vertical electric field system such as twisted nematic (TN) mode, wherein a pair of electrodes for applying an electric field, which drives liquid crystal molecules, are provided on different substrates; and a so-called horizontal electric field system typified by IPS mode, wherein pairs of electrodes are provided on the same substrate.
An example of an IPS mode liquid crystal display panel is shown in FIG. 14. A comb-shaped pixel electrode 2 and a comb-shaped common electrode 1 are disposed in a pixel region, which consists of a region surrounded by a pair of image signal lines 5 and a pair of scanning signal lines 6. The common electrode 1 is electrically connected to a common electrode bus bar 9. The pixel electrode 2 and the common electrode 1 are electrically insulated from each other, and when a switching element 7 electrically connects the image signal line 5 and the pixel electrode 2 in response to the scanning signal supplied via the scanning signal line 6, an electric field is formed between the pixel electrode 2 and the common electrode 1 in the manner shown in FIG. 15.
The IPS mode liquid crystal display panel has an advantage over vertical electric field liquid crystal display panels in that the IPS mode LCD panel shows smaller hue variation dependent on viewing angle. However, the IPS mode LCD panel has a lower aperture ratio than that of the vertical electric field system and therefore requires a high power backlight to ensure sufficient display image brightness. In order to improve pixel aperture ratio, Japanese Unexamined Patent Publication No. 9-61842 suggests that a first or a second electrode (i.e., pixel electrode 2 or common electrode 1) be made of a transparent conductor.
Note that, as shown in FIG. 15, an electric field is formed between the image signal line 5 and a common electrode 1a disposed adjacent thereto. Accordingly, the liquid crystal molecules located in the region designated by the slanted lines in FIG. 14 do not behave in the same fashion as do those located between the pixel electrode 2 and a common electrode 1b, but are driven by the electric field formed between the common electrode 1a and the image signal line 5. Therefore, this region does not exhibit a brightness that is required for the image to be displayed. In the operation of a liquid crystal display panel, generally, the polarity of the potential VS of the image signal line 5 is inversed with respect to the potential VC of the common electrode bus bar 9 so that the adjacent pixels or the adjacent lines have different polarities. For this reason, in cases where the polarity of the potential Vpix of the pixel electrode 2 differs from that of the potential VS, depending on various conditions such as shapes of the image signal line 5, the common electrode 1, and the pixel electrode 2, the gap of the pair of electrodes, the amplitude of VS, and so forth, the potential VS affects the electric field formed between the common electrode 1a and the pixel electrode 2 to change the orientation state of the liquid crystal molecules located between these electrodes. Thus, brightness variation is sometimes caused within a pixel, which prevents good image display.
In particular, when the screen is viewed from an oblique direction, light transmitting through this region does not go through the black matrix disposed in the panel, and thereby displayed images are adversely affected. This phenomenon greatly reduces the advantage of having a wide viewing angle that is attained by the horizontal electric field system liquid crystal display panel. It is possible to reduce the adverse effect by using a wide black matrix, but this technique reduces aperture ratio as a function of the relationship of the widths and the positions between the black matrix and the common electrodes.
In the prior art technique of the above-described publication, if the common electrode line 1a that is adjacent to the image signal line 5 is made of a transparent material, displayed image quality degrades because the region that shows a brightness different from a desired brightness is made to be a display region. That is, the mere use of a transparent electrode only exacerbates image quality degradation caused by this region. In view of this problem, by increasing the width of the common electrode 1a adjacent to the image signal line 5, for example, it is possible to reduce the adverse effect of the electric field formed between the image signal line 5 and the common electrode 1a, which influences the brightness of the pixel. However, this technique requires a large non-display region, lowering the aperture ratio, which is not particularly high in comparison with liquid crystal display panels of other display modes. To maintain brightness of the liquid crystal panel while maintaining the aperture ratio, the output power of the backlight needs to be increased. Consequently, an increase in power consumption is inevitable.
When the area per pixel is reduced to obtain higher resolution, the aperture ratio of the pixels accordingly decreases unless the widths of the pixel electrode and the counter electrode are reduced in proportion to the area. The above-mentioned prior art technique can reduce pixel size without decreasing the aperture ratio, but display quality is lowered because the region between the common electrode 1a and the image signal line 5 becomes a display region.
Japanese Unexamined Patent Publication No. 9-179096 proposes, as shown in FIG. 16, a liquid crystal display panel that has a conductive layer 50 covering portions of an image signal line 5 and a common electrode (reference electrode) 1, the conductive layer formed thereover and over an insulating layer (not shown). According to the publication, the electric field formed in the liquid crystal layer due to the potential of the image signal line 5 is shielded by the conductive layer 50 and therefore does not reach the common electrode 1. However, in this technique, since a capacitance is formed between the image signal line 5 and the conductive layer 50, undercharging caused by a large wiring time constant and consequent signal waveform deterioration occurs when the panel size is large and the image signal lines are accordingly long. Moreover, in cases where the insulating film has defects, such as pinholes, a short circuit between the image signal line and the conductive layer occurs, resulting in display defects.
It is an object of the present invention to provide a liquid crystal display panel that solves the foregoing problems of the prior art, has a high aperture ratio, and controls the image quality degradation caused by the electric field generated between the image signal lines and the common electrodes adjacent thereto.
A liquid crystal display panel according to the present invention has a line-shaped pixel electrode and a line-shaped common electrode disposed in each of pixel regions of the array substrate that is surrounded by a pair of image signal lines and a pair of scanning signal lines, and of these line-shaped electrodes, the electrodes that are disposed adjacent to and parallel the signal lines are made of an opaque conductor and at least one of the other electrodes is made of a transparent conductor.
To ensure a high aperture ratio, all the electrodes other than the ones adjacent to the signal lines are made of a transparent conductor.
The present invention may be applied to a so-called IPS mode liquid crystal display panel in which line-shaped pixel electrodes and common electrodes are provided on the array substrate side, as well as a liquid crystal display panel in which all or some of the common electrodes are provided on the counter substrate side.
In a preferred aspect of the invention, a portion of an electrode made of a transparent conductor is disposed on the array substrate so as to overlap, from the inside of the pixel, with a portion of an opaque electrode adjacent to an image signal line or a scanning signal line. That is, by disposing a transparent electrode on the side that contributes to good display, pixel aperture ratio is ensured and by disposing an opaque electrode on the signal line side, the light transmitting the region is shielded to suppress adverse effects of the region that influence display.
In another preferred aspect of the invention, an electrode adjacent to a signal line is provided with an inclined surface inclined toward the other side, i.e., toward an adjacent electrode. Thereby, a region in which electric flux lines are formed between the electrode and the adjacent electrode, i.e., a region that contributes to good display, is enlarged. For example, by using a combination type electrode in which portions of the electrodes overlap with each other, the electrode disposed in an upper layer is provided with an inclined surface in a region where the electrodes are overlapped.
In a further another preferred aspect of the invention, an electrode having a potential equal to that of the electrode adjacent to a signal line is disposed on the surface of the counter substrate so as to oppose the signal line. When the material of this electrode is a transparent conductor, display brightness is ensured. In addition, by providing this electrode so as to cover the electrode adjacent to the signal line, it is possible to suppress adverse effects of the potential of the signal line that influence the electric field formed between the electrode adjacent to the signal line and the electrode adjacent thereto.
In another liquid crystal display panel according to the present invention, as is the case with the foregoing, the line-shaped pixel electrode and common electrode are disposed in a pixel region, and in the centerline of the surface of the electrode that is disposed adjacent to and parallel to one of the image signal lines or one of the scanning signal lines, an electric flux line of an electric field formed between the electrode and an electrode adjacent thereto is inclined toward the adjacent electrode with respect to the direction of the normal of the array substrate.
In another preferred aspect of the invention, an electrode adjacent to a signal line has a surface inclined toward the other side, i.e., the adjacent electrode side. Thereby, a region in which electric flux lines are formed between the electrode and an electrode adjacent thereto, that is, a region that contributes to good display, is increased. For example, by employing a combination type electrode in which portions of the electrodes are overlapped such as described above, an inclined surface is formed in the region in which the electrodes are overlapped.
Still another liquid crystal display panel according to the invention has a light shielding member shielding a light transmitting a region between a signal line and an electrode adjacent thereto. The light shielding member is, for example, disposed on the array substrate side. When it is disposed in a layer lower than the electrode, the electrode formed in a peripheral region of the light shielding member is provided with an inclined surface.
The light shielding member is, for example, made of a conductive material, and more preferably, is electrically insulated from the surrounding component members.
In order to completely eliminate adverse effect of the light that transmits the region between a signal line and an electrode adjacent thereto, it is preferable that a black matrix is disposed so that it covers an area extending from the edge of the electrode adjacent to and parallel to one of the scanning signal lines or one of the image signal lines for a distance dxc2x7tan xcex8t, where d is the thickness of the liquid crystal layer and xcex8t is the angle formed by a direction of the normal of the surface of the array substrate and a proceeding direction of a light that has transmitted through the liquid crystal layer when the light undergoes total reflection at a boundary surface between the device and an outside.